Variation in microplastic characteristics during biosolid stabilization across wastewater resource recovery facilities in the United States and Canada

Microplastic (MP) particles have emerged as contaminants in biosolids globally, raising concerns about their potential ecological risks after land application. This study quantified MP particles in 50 biosolid samples (25 pre-stabilized, 25 post-stabilized) collected from 25 Wastewater Resource Recovery Facilities (WRRFs) across the United States and Canada. Three stabilization processes were evaluated, including Anaerobic Digestion (AD; 21 samples), Lime Stabilization (LS; 2 samples), and Incineration (INC; 2 samples). MP abundance, size distribution, shape, and polymer composition were characterized using standardized digestion, density separation, and micro-FTIR. MP particles after AD increased from 2030 to 2682 MP/g dw (+32.1 %). Small particles (20-150 μm) increased by 443 %, while those >600 μm decreased by 97 %, indicating strong fragmentation. Resistant polymers increased sharply, like PE, PS, and SR, while degradable polymers decreased, including PMMA and PEVA. LS reduced MP particles from 1388 to 964 MP/g dw (-30.5 %), yet fragmentation still occurred, with 20-150 μm particles increasing 442 % and particles >600 μm decreasing 97 %. Polymer shifts included increases in PE, ABS, and PU, and declines in PP, PVC and PEVA. INC lowered MP abundance from 2618 to 1909 MP/g dw (-27.1 %). Small particles increased by 277 %, and particles >600 μm decreased by 99 %. Heat-resistant polymers increased, including PS, PTFE, and PE, while sensitive polymers decreased, such as PMMA, PP and PEVA. Current biosolid stabilization methods are not specifically designed to address MP particles; however, the results of this study clearly show that MP particles become more fragmented and mobile through stabilization, particularly through AD and LS. This enhances the ecological and mobility risks of MP particles once introduced into the environment, particularly in agricultural soils.